Laser illumination systems and methods for dual-excitation wavelength non-linear optical microscopy and micro-spectroscopy systems

ABSTRACT

An illumination system is disclosed for providing dual-excitation wavelength illumination for non-linear optical microscopy and micro-spectroscopy. The illumination system includes a laser system, an optical splitting means, a frequency shifting system, and a picosecond amplifier system. The laser system includes a laser for providing a first train of pulses at a center optical frequency ω 1 . The optical splitting means divides the first train of pulses at the center optical frequency ω 1  into two trains of pulses. The frequency shifting system shifts the optical frequency of one of the two trains of pulses to provide a frequency shifted train of pulses. The picosecond amplifier system amplifies the frequency shifted train of pulses to provide an amplified frequency-shifted train of pulses having a pulse duration of at least 0.5 picoseconds.

SPONSORSHIP INFORMATION

This invention was made with government support under 1R01EB010244awarded by National Institutes of Health. The government has certainrights in the invention.

BACKGROUND

The invention generally relates to label-free imaging systems, andrelates in particular to non-linear optical microscopy andmicro-spectroscopy imaging systems employing efficient dual frequencylaser sources.

The development of confocal microscopy and genetically encodablefluorescent labels has transformed biological research. Labels however,may be perturbative of a sample, especially for imaging molecules thatare smaller than typical fluorophores (e.g., metabolites or drugs) andthat are not applicable for in vivo diagnostics due to toxicity and/ordelivery concerns. Certain molecules or properties (e.g., strain orstress in material science samples) cannot be labeled at all, andphotobleaching may be problematic for long term measurements.

Alternatively, vibrational spectroscopy may provide label-free chemicalcontrast based on intrinsic molecular properties of the sample. Yet, thetraditional techniques, infrared absorption (IR) and spontaneous Raman,are limited, and IR microscopy suffers from low spatial resolution andlimited imaging depth due to the long wavelength. Spontaneous Ramanmicroscopy has slow imaging speed due to the weakness of the signal.

Micro-spectroscopy generally involves capturing a spectrum from amicroscopic volume in a sample, while microscopy generally involvescapturing an intensity value as well as scanning such that multipleintensity values are captured to form picture elements (pixels) of amicroscopy image.

Infrared microscopy involves directly measuring the absorption ofvibrationally excited states in a sample, but such infrared microscopyis generally limited by poor spatial resolution due to the longwavelength of infrared light, as well as by a low penetration depth dueto a strong infrared light absorption by the water in biologicalsamples.

Raman microscopy records the spontaneous inelastic Raman scattering upona single (ultraviolet, visible or near infrared) continuous wave (CW)laser excitation. Raman microscopy has improved optical resolution andpenetration depth as compared to infrared microscopy, but thesensitivity of Raman microscopy is rather poor because of the very lowspontaneous Raman scattering efficiency (Raman scattering cross sectionis typically on the order of 10⁻³⁰ cm²). This results in long averagingtimes per image, which limits the biomedical application of Ramanmicroscopy.

Coherent Raman scattering (CRS) microscopy techniques, includingcoherent anti-Stokes Raman scattering (CARS) microscopy and stimulatedRaman scattering (SRS) microscopy allow signal amplification by up100,000× compared to spontaneous Raman, enabling label-free imaging withhigh temporal (imaging speeds up to video-rate, i.e., 30 frames/s) andsub-micron spatial resolution. Due to the use of nonlinear excitation,CRS microscopy is intrinsically three-dimensional, allowingnon-destructive optical sectioning of the sample. The excitation usesnear-infrared light within the optical window of biological specimen,allowing imaging depths up to a few hundred microns.

CRS is also free of photobleaching, if electronic resonances areavoided; and auto-fluorescence does not interfere, because it is notcoherently amplified. CRS imaging systems may be used in biology andmaterial science research, such as studying lipid metabolism, optimizingdrug formulation for trans-dermal delivery, and in biofuel production.Label-free microscopy is also being evaluated as a medical imagingmodality for delineation of tumor margins in brain and breast cancersurgery and early detection of melanoma.

Despite the advantages of CRS, high instrument cost and the technicalcomplexity limit its wider use and currently only a few laboratories canobtain high quality images. Providing laser sources for CRS ischallenging not only due to the comparative cost of laser systems ascompared to a complete conventional Raman system, but the illuminationmust be provided as two synchronized laser pulse-trains of picosecondpulse duration, with a tunable difference frequency to the precision ofa typical Raman line width.

Modulation transfer microscopy and spectroscopy imaging systems such asstimulated Raman scattering (SRS), spectral excitation of stimulatedRaman scattering (SRS Spectral), stimulated emission (SE), ground statedepletion (GD), photo-thermal (PT), two-color two-photon absorption(TPA), and stimulated Brillouin scattering generally involve reliance onthe non-linear interaction of two laser beams within a sample, anddetection of a characteristic, such as gain or loss, of one of theexcitation beams. This is in contrast to detecting a newly generated(new frequency) emission signal as is done, for example, in one-photonand two-photon excited fluorescence, spontaneous Raman scattering,coherent anti-Stokes Raman scattering (CARS), second harmonicgeneration, (SHG), sum frequency generation (SFG) and third harmonicgeneration (THG).

Such modulation transfer microscopy and micro-spectroscopy techniquesrequire a detection scheme that provides for detection of a relativelysmall signal (e.g., a small gain and loss signal) on top of noisylasers. This is generally achieved in accordance with variousembodiments based on modulation transfer—by modulating a feature of oneof the laser excitation beams and measuring the signal of interest withhigh sensitivity. In particular, the modulation transfers to the otherexcitation beam due to non-linear interaction within the sample, whichfacilitates detection of the signal of interest using a modulationsensitive detector. If the modulation frequency is chosen to be fasterthan the laser noise (e.g., greater than about 200 kHz), shot-noiselimited sensitivity may be achieved. Such modulation schemes are readilycompatible with beam-scanning microscopy and micro-endoscopy, video-rateimaging speeds, and multiplex excitation schemes.

An advantage of these non-linear optical imaging techniques as comparedto fluorescence microscopy, is that they allow for specific imagecontrast based on intrinsic spectroscopic properties of the sample,rather than extrinsic fluorescent labeling or dye staining. This isparticularly important for imaging of small molecules that can beperturbed by labeling and medical diagnostics because of a possibletoxicity of the used dyes. In CARS and SRS, chemical contrast is derivedfrom intrinsic molecular vibrations and in TPA, SE and GD microscopyfrom absorption properties of the molecules constituting the sample.

Common features of CARS and SRS include that each requires (1) pulsedlaser beams with a pulse-width shorter than about 10 ps, i.e., aspectral bandwidth of at least about 30 GHz, and (2) two synchronizedbeams that are overlapped in time.

Modulation transfer techniques further require that a property (such asintensity, polarization or time delay) of one of the beams is modulatedat a rate higher than 100 kHz allowing measurement the modulationtransfer from this modulated beam to the second, originally un-modulatedbeam due to the nonlinear interaction in the sample

These different techniques have different laser wavelength requirements.For CARS an SRS, the difference between the two excitation frequencies(|ω₁−ω₂|) is selected to be resonant with a vibrational frequency of thesample. The specific wavelengths of the two excitation fields, thereforeare not critical as long as the difference frequency is as desired. Suchsources are typically chosen to be in the range of about 700 nm to about1600 nm, for which biological samples are transparent. The tuning of thedifference frequency to a vibrational frequency of the sample (about 200cm⁻¹ to about 4000 cm⁻¹) should be to a precision of at least about 2nm.

Stimulated emission (SE) and ground state depletion (GD) microscopyinvolve tuning either ω₁ or ω₂ to be electronically resonant with thesample. With photo-thermal (PT) microscopy, either ω₁ or ω₂ is chosen tomatch the one or two photon electronic absorption frequency. Withtwo-color two-photon absorption (TPA), the sum of ω₁ and ω₂ is chosen tobe electronically resonant with the sample.

Many conventional laser systems for CARS and MTM techniques haveinvolved the use of mode-locked solid state lasers in order to achievethe pulse width shorter than 10 ps as such pulse-width that cannotconventionally be achieved with an electrically driven laser systems. Aparticular challenge, is the requirement of overlaps the pulses in timeprecisely (synchronization), as timing jitter translates into severenoise of the signal if it is bigger than the pulse width (i.e., muchsmaller than the required 10 ps).

Certain conventional implementations of CARS microscopy involved usingtwo Titanium Sapphire (Ti:Sa) lasers whose outputs were electronicallylocked to one another using feedback regarding the cavity length of oneof the lasers. Both Ti:Sa lasers were continuously tunable from about750 nm-1000 nm, which allowed imaging based on Raman frequencies in theentire spectral region from about 200 cm⁻¹-4000 cm⁻¹. Such systemshowever, suffered from timing jitter between the pulses, makinglong-term experiments impossible and limiting day-to-day stability ofthe system.

Later developed conventional system involved the use of opticalparametric oscillators (OPO) for label-free microscopy that areintrinsically locked due to synchronously pumping the OPO with the samelasers that provides the first beam. Such OPO laser systems may also bepumped with made-locked fiber lasers. The pump laser is typically fixedat 1064 nm and the OPO output is tunable from 750 nm to 1000 nm, againallowing to image any Raman band. The long-term stability, complexityand price of such OPO laser systems however, remains a shortcoming ofsuch systems. Moreover, dual frequency sources employing OPO lasersystems typically include an adjustable translation stage that ensuresthat the resulting two trains of laser pulses are temporally overlapped.Such an adjustable translation stage adjusts the optical path of one ofthe pulse trains within a short range to ensure temporal synchronicity.Variations in temperature of the imaging system will also affect thepath lengths and therefore synchronization.

Another approach to providing illumination systems for dual-excitationwavelength non-linear imaging systems has been based on time-lenslasers, which allow generation of pulses on demand with response to anelectronic trigger signal. A Yb time-lens laser may be triggered by aTi:Sa laser to provide to laser pulse trains for CRS with minimal timingjitter. One wavelength is fixed at 1040 nm and the other is tunable overthe entire region of Raman spectra. Again however, the synchronizationof the two oscillators is achieved electronically rather than by opticalseeding.

Other approaches to providing laser pulse trains for CRS have been basedon super-continuum generation (SC) in an optical fiber to generate afrequency shifted second train of pulses synchronized to the first trainof pulses. Typically SC spectra are very broad, much more broad than thetypical line-shape of Raman spectra and SC light sources for CRS arecombined with spectral compression schemes to recover the chemicalspecificity of CRS. This may be achieved either by spectral focusing CRSor by spectral compression via sum frequency generation.

This permits the generation of the spectral brightness required for CRSimaging and allows for fast imaging speed with pixel dwell times asshort as 4 μs. Because Raman SC spectra are very broad, SC light sourcesare broadly tunable (e.g., from 850 nm-1100 nm) and allow access Ramanpeaks across the full Raman spectrum. While relying on opticalsynchronization, this approach is different from the laser systemdisclosed herein in that the second pulse train is generated by supercontinuum generation in a nonlinear fiber rather than lasing; In thenonlinear fiber, the molecular population is unaffected similar to theparametric process in OPOs.

Ultrafast laser systems have also been disclosed based on seeding aYb-doped amplified with a super-continuum generated from an Er-dopedoscillator. Such systems however, only provide a single-color output,which is not suitable for CRS microscopy, which SRS requires thegeneration of two synchronized trains of picosecond pulses with narrowbandwidth (a few cm-1) and that are independently tunable over a widespectral range 800-3300 cm-1. Further, they are intrinsically broadbanddue to the broad spectral range from the super-continuum fiber thatdirectly seeds the amplifier.

There remains a need, therefore, for an efficient dual frequency lasersystem with reduced jitter for microscopy and micro-spectroscopy imagingsystems.

SUMMARY

In accordance with an embodiment, the invention provides an illuminationsystem for providing dual-excitation wavelength illumination fornon-linear optical microscopy and micro-spectroscopy. The illuminationsystem includes a laser system, an optical splitting means, a frequencyshifting system, and a picosecond amplifier system. The laser systemincludes a laser for providing a first train of pulses at a centeroptical frequency ω₁. The optical splitting means divides the firsttrain of pulses at the center optical frequency ω₁ into two trains ofpulses. The frequency shifting system shifts the optical frequency ofone of the two trains of pulses to provide a frequency shifted train ofpulses. The picosecond amplifier system amplifies the frequency shiftedtrain of pulses to provide an amplified frequency-shifted train ofpulses having a pulse duration of at least 0.5 picoseconds. Inaccordance with further embodiments, the picosecond amplifier system maybe a narrowband amplifier system or a chirped amplifier system.

In accordance with another embodiment, the invention provides anillumination system includes a laser system, an optical splitting means,a frequency shifting system, an amplifier system, and combining means.The amplifier system is for amplifying the frequency-shifted train ofpulses to provide an amplified frequency-shifted train of pulses, andthe combining means is for combining the amplified frequency-shiftedtrain of pulses with a second of the two trains of laser pulses from theoptical splitting means to provide the amplified frequency-shifted trainof pulses and the second of the two trains of laser pulses from theoptical splitting means as a collinear train of laser pulses for thedual-excitation wavelength illumination

In further embodiments, the illumination system is provided in adual-excitation wavelength nonlinear Stimulated Raman Scatteringmicroscopy or two-color two-photon microscopy system.

In accordance with a further embodiment, the invention provides a methodof providing dual-excitation wavelength illumination for non-linearoptical microscopy and micro-spectroscopy. The method includes the stepsof providing a first train of pulses at a center optical frequency ω₁using a laser oscillator; dividing the first train of pulses at thecenter optical frequency ω₁ into a first split train of pulses and asecond split train of pulses; shifting the optical frequency of thefirst split train of pulses to provide a frequency shifted train ofpulses; and amplifying the frequency shifted train of pulses to providea amplified frequency-shifted train of pulses having a pulse duration ofat least 0.5 picoseconds.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description may be further understood with reference tothe accompanying drawings in which:

FIG. 1 shows an illustrative diagrammatic view of an illumination systemand imaging system employing a dual-frequency source in accordance withan embodiment of the invention;

FIGS. 2A and 2B show illustrative diagrammatic views of narrowbandexcitation and output spectra in an SRS system in accordance with anembodiment of the present invention;

FIGS. 3A and 3B show illustrative graphical representations of estimatedsignal to noise ratios and required average power requirementsrespectively for pulse parameters in a CRS system;

FIG. 4 shows an illustrative diagrammatic view of a an illuminationsystem in accordance with an embodiment of the invention;

FIG. 5 shows an illustrative diagrammatic view of a an laser system foruse in the illumination system of FIG. 4 in accordance with anembodiment of the invention;

FIG. 6 shows a illustrative diagrammatic view of a portion of theillumination system of FIG. 4 employing a frequency shifting system inaccordance with an embodiment of the invention;

FIG. 7 shows a illustrative diagrammatic view of a portion of theillumination system of FIG. 4 employing a further frequency shiftingsystem in accordance with another embodiment of the invention;

FIGS. 8-11 show illustrative diagrammatic views of a portion of theillumination system of FIG. 4 employing further narrowband amplifiersystems in accordance with further embodiments of the invention;

FIG. 12 shows an illustrative diagrammatic view of a portion of theillumination system of FIG. 4 employing a plurality of amplifiers inaccordance with an embodiment of the invention;

FIGS. 13-14 show illustrative diagrammatic views of a portion of theillumination system of FIG. 4 employing a plurality of amplifiers inaccordance with further embodiments of the invention;

FIGS. 15-16 show illustrative diagrammatic views of a portion of theillumination system of FIG. 4 employing frequency doubling/triplingunits in accordance with further embodiments of the invention;

FIGS. 17-19 show illustrative diagrammatic views of an illuminationsystem in accordance with further embodiments of the invention employingvarious laser systems;

FIG. 20 shows an illustrative diagrammatic view of an illuminationsystem in accordance with further embodiments of the invention employinga Erbium dopes fiber oscillator, as well as Erbium-doped andYtterbium-doped power amplifiers;

FIGS. 21A-21B show illustrative graphical representations of tuningranges for illumination sources in accordance with embodiments of thepresent invention employing different gain mediums;

FIG. 22 shows an illustrative graphical representation of therelationship between frequency and time, showing changes in the pulsesover time and frequency that permits spectral focusing of the excitationillumination in accordance with an embodiment of the invention;

FIG. 23 shows an illustrative diagrammatic view of a portion of theillumination system of FIG. 4 employing a chirped amplifier system inaccordance with an embodiment of the invention;

FIG. 24 shows an illustrative diagrammatic view of a portion of theillumination system of FIG. 4 employing a chirped amplifier system inaccordance with another embodiment of the invention;

FIGS. 25-30 show illustrative diagrammatic views of a portion of theillumination system of FIG. 4 employing a chirped amplifier system inaccordance with further embodiments of the invention;

FIG. 31 shows an illustrative photographic representation of a CRS imageof polystyrene beads obtained using an illumination system of thepresent invention;

FIG. 32 shows an illustrative graphical representation of gray valueintensities along a cross-section of the photographic representation ofFIG. 31;

FIGS. 33A and 33B show illustrative photographic representations of CH2vibrations (lipids) obtained using an illumination system of the presentinvention and of CH3-vibrations (proteins) obtained using anillumination system of the present invention; and

FIG. 34 shows an illustrative graphical representation of an image forsebaceous land in a subject.

The drawings are shown for illustrative purposes only.

DETAILED DESCRIPTION

The promise of label-free microscopy is that one may obtain rich,chemical specific contrast based on intrinsic properties of the sample.As each type of chemical bond has a specific stiffness (e.g., C═C isstiffer than C—C) and associated mass (e.g., C—C is heavier than C—H),it has a characteristic vibrational frequency Ω. Vibrational spectra ofthe sample, which consist of the vibrational frequencies of themolecule, provide a unique molecular fingerprint. Raman scattering is anelegant way to measure vibrational spectra with visible light. When amolecule is excited (de-excited), an incident photon is annihilated anew red-shifted (blue-shifted) photon is generated at the Stokes(anti-Stokes) frequency ω_(S)=ω_(P)−Ω (and ω_(AS)=ω_(P)+Ω) due to energyconservation. The emission spectra, σ(Ω)=σ(ω_(P)−ω_(S)), can be measuredby dispersing the light on a spectrometer.

In a specific implementation, the invention provides an ultrafastdual-excitation wavelength laser source based on a fiber lasertechnology. Because light is guided within the optical fiber,misalignment is impossible. Existing fiber-lasers do not, however, reachthe same performance level of free-space systems in CRS. An importantrealization is that the difference frequency of the two most commonfiber gain media, erbium (Er) and ytterbium (Yb), coincides with thehigh-wavenumber region of Raman spectra, where most CRS microscopy isperformed. The invention provides an all-fiber system based on opticalsynchronization of Er- and Yb-doped power amplifiers via super-continuumseeding and careful control the pulse properties for CRS microscopy.

FIG. 1 shows an illumination system 10 in accordance with an embodimentof the invention together with a microscopy imaging system 12. Theillumination system 10 includes a laser system 14 (that provides twotrains of laser pulses 16, 18) and a combiner system 20 that combinesthe two trains of laser pulses such that they are collinear andspatially and temporally overlapped. One of the trains of laser pulses16 is at a center frequency ω₁ (e.g., a Stokes frequency of about 1030nm), and the other train of laser pulses 18 is at a different centerfrequency (e.g., 800 nm).

The combined trains of laser pulses 22 are directed via a scanhead 24(that scans in mutually orthogonal x and y directions), into amicroscope 26 that includes optics 28 that direct and focus the combinedtrains of laser pulses into the focal volume, e.g., via a mirror 30. Theillumination from the focal volume is directed by a condenser 32 onto anoptical detector 34. One of the trains of laser pulses 16 or 18 (a firsttrain of laser pulses) is modulated within the laser system responsiveto a modulation signal 36, and, at the detector 34 the modulated firstbeam (e.g., the Stokes beam) is blocked by an optical filter 38. Theoptical detector 34, such as a photodiode, therefore measures theintensity of the other (second) beam (e.g., the pump beam) only.

The system may further include balanced detectors. In particular,another optical detector 40 may be employed with a filter 41 such thatthe detector 40 only sees the original pump or Stokes beam that was notmodulated. The difference between the outputs of these detectors 34 and44 is provided by a subtraction unit 43, which outputs the outputelectrical signal 46. A delay unit 42 permits adjustment of the timingof the original second train of laser pulses. An electrical outputsignal 46 is provided to a signal processor 48.

The first train of laser pulses is modulated at modulation frequency f,by a modulation system that includes, for example, a modulator withinthe laser system as discussed in further detail below, a controller 50,and a modulation source 52. The modulation source provides a commonmodulation control signal to the controller 50 as well as to a signalprocessor 48. The integrated intensity of substantially all frequencycomponents of the second train of laser pulses from the optical detector34 is provided to the signal processor 48, and the modulation (amplitudeand/or phase) of the integrated intensity of substantially all theoptical frequency components of the second train of laser pulses due tothe non-linear interaction of the first and second trains of laserpulses in the focal volume is detected at the modulation frequency f toprovide a pixel of an image to a microscopy control computer 54. Themicroscopy control computer 54 is employed as an imaging system, andfurther provides user control of the scanhead 24 as shown at 56.

In a further embodiment, an epi-directed detection scheme may beemployed wherein the illumination from the focal volume is received backthrough the optics 28 and passes through a filter 62 to a detector 60that provides an electrical output signal to a subtraction unit 68. Thesubtraction unit 68 also receives an output signal from a detector 58via a filter 66 that receives the original pump or Stokes beam that wasnot modulated. Again, a delay unit 64 permits adjustment of the timingof the original second train of laser pulses. The use of the seconddetector and the subtraction unit provides that any laser backgroundnoise as well as any low frequency variations in the laser power, willbe removed from the detected signal, (whether detected in the forward orepi direction).

Tuning control of the lasers output trains may also be provided usingthe microscopy control computer 54 that directs a control signal 70 tothe laser system 14 as shown. Such tuning control may control thefrequency difference between the pump and Stokes beams to provide fortuning into different compositions in the sample. Coupled with theability to scan the excitation fields (as shown at 74), the controlcomputer 54 may then direct the microscopy system to scan an area for avariety of different compositions, and the resulting pixel data isprovided (as shown at 74) to the control computer 54.

The modulation system may provide amplitude modulation of the first beamto provide a modulated pulse train such that only alternating pulses ofthe first pulse train are coincident with the pulses of the second pulsetrain. Such amplitude modulation of the first beam may be achieved usinga Pockel cell and polarization analyzer as the modulator, and a Pockelcell driver as the controller.

If the modulation rate is of the same order of the repetition rate ofthe laser, countdown electronics must be utilized to guarantee thesynchronization (phase) between the modulation and the pulse train. Awide variety of different modulation rates are also possible. In furtherembodiments, the contrast pulses may have an amplitude that issubstantially zero by switching off the pulses at the modulationfrequency, for example using an electro-optic modulator (such as a MEMsdevice or a galvanometric scanner) or an acousto-optic modulator.

Amplitude modulation of the pump or Stokes pulse trains may therefore beachieved, and the increase of the Stokes pulse train or decrease of thepump pulse train may be measured. By modulating the pump train of pulsesand then detecting the Stokes train of pulses from the focal volume,Raman gain may be determined by the processing system. In an embodiment,the pump beam may be modulated, the Stokes beam may be detected from thefocal volume, and Raman gain may be determined by the processing system.In a further embodiment, the Stokes beam may be modulated, the pump beammay be detected from the focal volume, and Raman loss may be determinedby the processing system.

In spontaneous Raman scattering, the sample is excited with light at asingle frequency ω_(p). The output spectrum contains new radiation onboth the Stokes (ω_(S)) and anti-Stokes sides (ω_(nS)) due to inelasticlight scattering off molecular vibrations. In CRS, the combined actionof pump and Stokes beams effectively transfers the molecules in thesample from the ground state into the targeted vibrational state. As aconsequence a pump photon is absorbed and a Stokes photon is generated.This allows signal amplification by up 100,000× compared to spontaneousRaman scattering.

In contrast to fluorescence, the energy of the incident photonstypically does not match an electronic excited state, and spontaneousRaman scattering is mediated through a virtual state rather than anexcited electronic state, relying on vacuum fluctuations to generate thenew emission. As such, spontaneous Raman scattering is extremely weak,resulting in long averaging times to obtain high signal to noise ratio(SNR) spectra and slow imaging speed in microscopy.

In CRS the sample is excited with two laser beams. The differencefrequency, Δω=ω_(P)−ω_(S), is tuned to match the frequency of a targetvibration, Ω. In this case, the transition from the virtual state intothe vibrational excited state is stimulated, not spontaneous, similar tothe well-known phenomena of stimulated emission, which allows for lightamplification in lasers. The molecular transition rate is consequentlyenhanced by r_(coh)/r_(spo)=n_(Stokes)+1, where n_(Stokes) correspondsto the number of photons in the optical mode of the Stokes beam and the+1 indicates spontaneous transitions. This is the origin for signalenhancement in CRS and the basis of fast label-free imaging. Ultimatelyn_(Stokes) and thus enhancement is limited by photodamage to ˜100,000×in biological specimen. This means that a complete high-resolutionimaged may be acquired with CRS in the same time as a single spectrumwith spontaneous Raman. Optimization of the laser parameters enables forhigh sensitivity and imaging speeds up to video-rate.

The two most popular CRS techniques, CARS and SRS, share this commonexcitation scheme and are, in fact, excited simultaneously. They differin the detection. The CARS signal at the anti-Stokes frequency isdetected by blocking the excitation beam with a high optical density(OD) filter after the sample. SRS, comprising stimulated Raman gain(SRG) of the Stokes beam and stimulated Raman loss (SRL) of the pumpbeam, are the intensity variations ΔI of the excitation beams Iassociated with the energy transfer from the optical field to thesample. Under biomedical excitation condition, the relative changes arehowever small (ΔI/I<10⁻⁴) and can be buried in the intensityfluctuations of the excitation lasers or the linear absorption orscattering of the sample.

A high-frequency modulation/detection scheme has therefore beendeveloped to extract the SRS signal with high sensitivity as disclosed,for example, in U.S. Pat. No. 8,027,032, the disclosure of which ishereby incorporated by reference in its entirety. SRS is more sensitivethan CARS, linear in the concentration of the target molecule and freefrom artifacts due to phase-matching. Detection is, however, somewhatmore challenging. In particular, signal detection in reflection ofthick, non-transparent samples (epi-detection) is more straight forwardwith CARS. As CARS and SRS share the same excitation conditions, theillumination systems of the present invention are applicable to both CRStechniques.

The input trains of laser pulses should be narowband (with a pulse thatis longer than 0.5 ps). With reference to FIG. 2A both of the twosynchronized trains of laser pulses 80, 82, may be narrowband pulses,and as shown at 86 and 88, the increase in intensity of the Stokesillumination (ΔI_(S)) may be detected or the decrease in the pumpillumination (ΔI_(p)) may be detected. As also shown in FIG. 2B,ω_(p)−ω_(S)=Ω. When both are narrowband, the difference frequency has tobe adjusted, for example, by tuning either of the two centerfrequencies.

For CRS, the illumination system 10 should provide the two trains oflaser pulses ω_(p) and ω_(S), one of which is modulated, and CRS occursas long as Δω=ω_(p)−ω_(S) falls within the linewidth, typically ˜20 cm⁻¹(at 800 nm, 20 cm⁻¹=1.3 nm) of a Raman transition. As a consequence, aCRS laser source has to fulfill the following requirements. First, atleast one of the laser beams has to be tunable to a precision narrowerthan about 0.2 nm. Second, the laser bandwidth has to be narrower thanthe typical Raman line-width (<3 nm). Third, the absolute wavelength isnot critical, in contrast to spontaneous Raman scattering, which scaleswith 1/λ⁴. For biological samples however, it is advantages to excitethe sample in the optical window from 700-1300 nm, where both scatteringand absorption are minimal and resolution with a high numerical aperture(NA) lens is sub-cellular.

Achieving maximal sensitivity is a primary challenge for CRS microscopy.This is where most laser systems that fulfill the first and thirdcriteria above fail. It is critical to consider thesignal-to-noise-ratio (SNR) for various laser parameters. For continuouswave (CW) lasers, the SRS signal is proportional to the product of theaverage power of the pump and Stokes beams, i.e., SRS signal isnonlinear in the overall excitation power.

It is therefore advantageous to utilize pulsed lasers, which have highpeak powers but moderate average powers to minimize heating effects inthe sample due to linear absorption. For pulsed lasers with averagepower Ï_(p,S), pulse duration duration τ and repetition rate R, SRSsignal is proportional to the following:(Î _(p) /τ·R)·(Î _(S) /τ·R)·τ·R=Î _(p) ·Î _(S) /τ·R

High-frequency, phase-sensitive detection of SRS is close to shot-noiselimited, i.e. noise for SRL is proportional to Ï_(p) ^(0.5) andSNR∝Ï_(S)·Ï_(p) ^(0.5)/τ·R. For a fixed total average power Î at thesample, it is thus advantageous to chose Î_(S) such that

${\hat{I}}_{S} = {{2{\hat{I}}_{p}} = {\frac{2}{3}\hat{I}}}$in order to maximize the SNR. A similar argument can be made for CARSmicroscopy, and it can be shown that for CRS microscopy,SNR∝Î^(1.5)/τ·R.

The use of a low repetition rate, femtosecond laser system wouldmaximise the SNR, but a hard limit exists however, on the repetitionrate for fast microscopy, that is set to R>10 MHz (and R>40 MHz forvideorate imaging) by the fact that at least one laser pulse is requiredper pixel (and more if the laser repetition rate is not synchronized toof the pixel clock of the microscope). The pulse duration is limited bythe time-bandwidth product, which states that a laser pulse of a givenduration can only be achieved if it has a certain spectral bandwidth.Requirement (B) limits τ>0.5 ps. This requirement differentiates CRSfrom two-photon fluorescence, which is typically excited withfemtosecond (fs) lasers.

Further, it is also important to consider the damage threshold of thesample. Optimizing the sensitivity for CRS means designing the CRS lasersystem to approach the damage threshold. While absolute quantificationof photodamage is sample and metric dependent, studies suggest thatnear-IR laser damage in biological samples is primarily due to nonlinearabsorption phenomena with a scaling of I_(ave) ^(γ)/(τ·R)^(γ-1) andnonlinear scaling parameter γ being in the range from 2.5 to 3.5. Theapproximate measurements carried out with R=80 MHz, NA=1.2 andλ_(pump)=817 nm and Stokes λ_(Stokes)=1064 nm, indicate that the sampleshows morphological changes after a single scan (the most drastic formof damage) for Î=25 mW at 180 fs, Î=80 mW at 1 ps, and Î=280 mW at bps.This suggests that γ≈3.2, i.e. the photodamage is more nonlinear thanthe CRS SNR. The maximal average power can thus be estimated by theexperimental equation:

${\hat{I}}^{\max} \approx {25\mspace{14mu}{{mW} \cdot ( {{\tau/180}{{fs} \cdot {R/80}}\mspace{14mu}{MHz}} )}\frac{3.2 - 1}{3.2}} \approx {25\mspace{14mu}{{mW} \cdot ( {{\tau/180}{{fs} \cdot {R/80}}\mspace{14mu}{mHz}} )^{0.7}}}$

With the above assumptions, simulations were provided to determine thebest pulse properties for CRS. It has been found that and found thatwithin the hard limits (τ=1 ps-10 ps and R>20 MHz), the CRS SNR hardlyvaries with the exact parameter order to achieve the same signal with a10-ps rather than 1-ps laser pulse however, the average powerrequirements of the laser system is increased ˜5×. Ideally the user canspecify the system depending on whether it is to be optimized for highspectral resolution (i.e., larger τ and higher Î) or low average power,as for medical applications (i.e., shorter τ, lower R, and lower Î).

FIG. 3A shows at 90 an estimated SNR as a function of pulse duration ata fixed repletion rate of 40 MHz and normalized to SNR for excitationwith 80 HMz, 6 ps pulses. This assumes excitation with maximal averagepower that does not cause photo-damage, and neglects effects due tolimited Raman line-width. FIG. 3B shows at 92 an average powerrequirement for the laser system to achieve maximal SNR.

In summary, an ideal CRS laser system further requires a pulsed laserwith pulse duration of 0.5-10 ps, repetition rate of 20-100 MHz, andsufficient average power given a certain pulse duration and repetitionrate (FIG. 2B), as well as temporal synchronization of the pulse trainsto a fraction of the pulse duration to avoid intensity fluctuations dueto timing jitter.

In addition to the narrowband approach based on picosecond (ps) lasers,other modes of CRS microscopy are being investigated. A technique knownas spectral focusing uses broadband frequency-chirped pulses, i.e.,laser pulses with a center frequency that varies over time. If the chirprate of both pump and Stokes pulses are matched, the frequencydifference does not vary and is effectively narrowband, even though theabsolute frequency is swept. Tuning the time delay changes thedifference frequency, allowing different Raman peaks to be targeted.Spectral focusing is attractive for fast and reproducible spectraltuning or frequency modulation schemes.

Other approaches are based on exciting multiple vibrationssimultaneously. In multiplex CRS, either the pump or Stokes beam isbroadband, while the other is narrowband. By performing excitation oremission spectroscopy, signal from each vibration is detectedseparately. This approach allows for high chemical specificity andsimultaneous multi-color imaging at a reduced imaging speed. There is astrong need for better laser sources for multiplex SRS, as mostdemonstrations are based on unstable electronic locking of fs- andps-lasers.

The disclosed laser system can also be applied to other spectroscopiesand label-free microscopy techniques, such as two-color, two-photonabsorption (TPA) microscopy, stimulated emission (SE) microscopy,ground-state depletion (GD) microscopy and sum-frequency generation(SFG). These techniques have slightly different requirements on thelaser system. Nevertheless, they share the common feature of two-colorexcitation with pulsed lasers.

TPA requires the sum-frequency of the two beams to be resonant with anelectronic state of the sample (e.g., hemoglobin or melanin) and usuallyutilizes fs beams to further probe excited state dynamics. TPA iscurrently being explored for early detection of melanoma. SE/GD requiresat least one of the two lasers to be tuned into an electronic absorptionstate of the sample.

All modulation transfer techniques (SRS, TPA, and SE/GD) further requirea property (such as intensity, polarization or time delay) of one of thebeams is modulated at a rate higher than 100 kHz allowing measurementthe modulation transfer from this modulated beam to the second,originally un-modulated beam due to the nonlinear interaction in thesample.

The invention involves providing an illumination system for an imagingsystem such as a CRS imaging system that is economical and efficient tomanufacture, and provides in particular, a CRS laser system design thatis based on optical synchronization of two laser amplifiers. Theapproach starts with a laser oscillator with the first center frequencyω1.

This first laser is either a high-power laser (e.g., solid state Ti:Salaser) or a low-power laser (e.g., fiber oscillator), which can beamplified to high power in consecutive steps (e.g., see reference forspecific implementation). The output is split with an optical splitter.Part of the output provides the first pulse train for Coherent RamanScattering Microscopy (i.e., either the pump or Stokes beam) orModulation Transfer Microscopy (i.e., either the pump or probe beam).The other part of the output is fed into a frequency shifting unit,which generates optically synchronized light at a second opticalfrequency ω₂. The frequency shifting may for example, be achieved bysuper-continuum generation in a high nonlinear fiber (HNLF). This newlight at ω₂ is then used to seed a laser amplifier at this frequency.

FIG. 4, for example, shows an illumination system that includes a lasersystem 14 and a combiner system 20. The laser system 14 includes a lasersource 100 that provides a first train of laser pulses. The first trainof laser pulses is divided, and a portion is provided to a frequencyshifting system 102 and then to a picosecond amplifier system 104. Amodulator 106 may be employed to modulate the train of pulses responsiveto a modulation signal 36 (as discussed above), and in certainembodiments, the other signal may be modulated by a modulator 108. Thecombiner system 20 may include a delay unit 110 that provides anadjustable delay to ensure that the first (e.g., Stokes) and second(e.g., pump) trains of laser pulses are temporally coincident with oneanother. The tuning control signal 70 is coupled to the picosecondamplifier system 104 for providing tuning control of the differencefrequency between the excitation trains of laser pulses.

The laser source system may be a high power oscillator, or in otherembodiments as shown in FIG. 5, the laser source system 120 may includea low power oscillator 122 and an amplifier 124 for providing the firsttrain of laser pulses.

The frequency shifting system 102 may include a highly non-linear fiber,or with reference to FIG. 6 may include a frequency shifting system 134that includes an amplifier 136 and a highly non-linear fiber 138 (e.g.,a photonic crystal fiber). The laser system 132 of the system 130provides a first train of laser pulses 144 as discussed above that iscombined with a second train of laser pulses 142 from the picosecondamplifier system 140.

The picosecond amplifier system may include a narrowband amplifiersystem. FIG. 7, for example shows a system 150 that includes a lasersystem 158 for providing a first train of pulses 164, a frequencyshifting system 160, and a narrowband amplifier system 152 that includesa narrowband transmission filter 154 and an amplifier 156 for providinga second train of laser pulses 162. In other embodiments, the positionsof the amplifier 156 and narrowband transmission filter 154 may bereversed.

In accordance with a further embodiment and with reference to FIG. 8, asystem 190 may include a laser system 192 for providing a first train ofpulses 194, a frequency shifting system 196, and a narrowband amplifiersystem 198 that includes a circulator 200, an amplifier 202 and anarrowband transmission filter 204. The second train of laser pulses 206is provided from the circulator 200 as shown.

In accordance with a yet a further embodiment and with reference to FIG.9, a system 210 may include a laser system 212 for providing a firsttrain of pulses 214, a frequency shifting system 216, and a narrowbandamplifier system 218 that includes a first amplifier 220, a narrowbandfilter 222, and a second amplifier 224 that provides the second train ofpulses 226. In accordance with a further embodiment and with referenceto FIG. 10, a system 230 may include a laser system 232 for providing afirst train of pulses 234, a frequency shifting system 236, and anarrowband amplifier system 238 that includes a first amplifier 240, acirculator 242, a narrowband filter 244, a second amplifier 246 and aminor 248. The second train of laser pulses 250 is provided from thecirculator 242 as shown.

In accordance with a further embodiment and with reference to FIG. 11, asystem 260 may include a laser system 262 for providing a first train ofpulses 264, a frequency shifting system 266, and a narrowband amplifiersystem 268 that includes a circulator 270, first amplifier 272, anarrowband reflective filter 274, and a second amplifier 276. The secondtrain of laser pulses 278 is provided from the circulator 270 via thesecond amplifier 276 as shown.

Additional embodiments for the illumination system include thefollowing. As shown in FIG. 12, the system 280 may include a lasersystem 282 that provides a first train of pulses 286 via an amplifier284, and a frequency shifting system 288 and narrowband amplifier system290 for providing the second train of laser pulses 292. As shown in FIG.13, the system 300 may include a laser system 302 that provides a firsttrain of pulses 308 via a narrowband filter 304 and an amplifier 306,and a frequency shifting system 310 and narrowband amplifier system 312for providing the second train of laser pulses 314. As shown in FIG. 14,the system 322 may include a laser system 324 that provides a firsttrain of pulses 330 via a first amplifier 324, a narrowband filter 326and a second amplifier 328, as well as a frequency shifting system 330and a narrowband amplifier system 332 for providing the second train oflaser pulses 334.

Additional embodiments of the invention include the following systemsthat employ frequency multiplication units. FIG. 15, for example shows asystem 340 that includes a laser system 342 that provides a first trainof pulses 344 via a frequency doubling or tripling system 346, as wellas a frequency shifting system 348 and a narrowband amplifier system 350for providing a second train of laser pulses 352. FIG. 16 shows a system360 that includes a laser system 362 that provides a first train ofpulses 364, as well as a frequency shifting system 366, a narrowbandamplifier system 368, and a frequency doubling or tripling system 370for providing a second train of laser pulses 372.

In accordance with various embodiments, the gain media for systems ofthe invention may include a variety of doped material. For example, andas shown in FIG. 17, the system 380 may include an Erbium-doped lasersystem 382 for providing a first train of laser pulses 384 at 1530nm-1610 nm (e.g., Stokes beam), as well as a frequency shifting system388 and a Ytterbium amplifier system 390 for providing a second train oflaser pulses 392 at 1010 nm-1080 nm (e.g., pump beam). In accordancewith an embodiment, the system 380 may optionally further include anErbium-doped amplifier system 386 for amplifying the first train oflaser pulses as shown. The pump frequencies for the erbium-doped gainmaterial may be about 980 nm, about 1480 nm or about 1550 nm (in-bandpumping).

In further embodiments, the invention may provide a system 400 as shownin FIG. 18 that includes an Erbium-doped laser system (1530 nm-1610 nm),and an Erbium-doped amplifier system (1010 nm-1080 nm) and a frequencydoubling system 408 for providing the first train of laser pulses (e.g.,pump beam). The system 400 also includes a frequency shifting system 410and a Ytterbium amplifier system 412 for providing the second train oflaser pulses 414 at 1010 nm-1080 nm (e.g., Stokes beam).

In accordance with further embodiments, the invention may provide asystem 420 shown in FIG. 19 including a Ytterbium-doped laser system 422(1010 nm-1080 nm), and a Ytterbium-doped amplifier system 424 (1010nm-1080 nm) for providing a first train of laser pulses (e.g., a Stokesbeam). The system 420 also includes a frequency shifting system 428, anErbium-doped amplifier system 430 (1530 nm-1610 nm) and a frequencydoubling unit 432 for providing a second train of laser pulses 434(e.g., a pump beam).

With reference again to FIG. 17, in accordance with further embodiments,the laser system 382 may instead be a Ytterbium-doped laser system (1010nm-1080 nm) and the amplifier 386 may be a Ytterbium-doped amplifiersystem (1010 nm-1080 nm) for providing, e.g., a Stokes beam. Theamplifier system 390 may be a semiconductor amplifier system (700 nm-900nm) that follows the frequency shifting system to provide the secondtrain of pulses (e.g., pump beam.

In yet further embodiments, and again with reference to FIG. 17, thelaser system 382 may be a titanium:sapphire (Ti:Sa) laser system (750nm-950 nm) for directly providing a first train of laser pulses (e.g.,pump beam), and the amplifier 390 may be a Ytterbium-doped amplifiersystem (1010 nm-1080 nm) following the frequency shifting system 388 forproviding the second train of laser pulses (e.g., Stokes beam). Infurther embodiments, the laser system 382 may be a Thulium fiberamplifier (1800 nm-2100 nm) or a Holmium fiber amplifier (1800 nm-2100nm), each of which may be shiftable to 900 nm-1050 nm.

In an implementation of an all fiber illumination system of theinvention in which the output from an Erbium-doped fiber-oscillator issplit into two arms to seed Erbium-doped and Ytterbium-doped poweramplifiers. Optical synchronization is provided by frequency shiftingusing supercontinuum (SC) generation in a highly nonlinear fiber (HNLF).Tunable narrowband output is achieved with an in-line filter. Detaileddesign criteria are discussed in the research plan.

In particular, the illumination system 440 of FIG. 20 includes a lasersource system 442, an amplifier 444, and a frequency doubling unit 446for providing a first train of laser pulses 448 (e.g., a pump beam). Thesystem 440 also includes frequency shifting system 450 and an amplifiersystem 452 for providing a second train of pulses 454 (e.g., a Stokesbeam). The laser source system 442 may be an oscillator that includes awavelength division multiplexer 460 that receives pump illumination (980nm) as shown at 462, an isolator 464, a carbon nanotube saturableabsorber 466, an output coupler 468, another isolator 470, a 50/50 fibersplitter 472, and a modulator 474.

The first train of laser pulses from the oscillator is provided to anErbium power amplifier 444 that includes wavelength division multiplexerthat receives pump illumination (e.g., 1480 nm) as shown at 482, anErbium-doped fiber (having normal dispersion) 484 and a compressionfiber 486. The output of the Erbium power amplifier 444 is provided to adoubling crystal 446, which provides the first output train of laserpulses (pump beam) at 790 nm.

The second train of laser pulses from the oscillator is provided to thefrequency shifting unit 450 that includes a wavelength divisionmultiplexer 490 that receives pump illumination (e.g., 976 nm) as shownat 492, an Erbium-doped fiber (normal dispersion), a compression filter496, a highly non-linear fiber 498 and a Ytterbium-doped fiber 500. Theoutput of the frequency shifting unit 450 is provided to a Ytterbiumpower amplifier 452 that includes a tunable narrowband filter 502, awavelength division multiplexer 504 that receives pump illumination(e.g., 980 nm) as shown at 506, and a Ytterbium-doped fiber 508, whichprovides the second output train of laser pulses (Stokes beam) at 1010nm-1080 nm. The first and second trains of laser pulses 448, 454 arecombined as discussed above.

The Erbium fiber oscillator with a center frequency of about 1550 nm andfrequency bandwidth of about 10 nm is therefore employed, and thisfrequency is shifted to about 1040 nm to seed the Ytterbium fiberamplifier. Amplification of the Erbium signal may be performed before orafter the optical splitter. Further, the Erbium signal may also befrequency doubled or tripled to achieve shorter wavelengths as preferredfor microscopy. Center frequencies can further be shifted using passiveor tunable optical filters or by gain engineering (e.g., in-bandpumping).

Typically, CRI systems are based on narrowband, picosecond lasers. Thismeans, that the laser frequency bandwidth is narrower than the typicalbandwidth of Raman transitions (e.g., 1 nm). In the above laser systemthis may either be achieved by using a laser oscillator with narrowfrequency bandwidth or by using a broad-band laser system in combinationwith narrowband optical filters, which can be fixed frequency ortunable. The reduced intensity by frequency filtering can be restored byadditional laser amplifiers.

An alternative to narrowband CRS is a technique known as spectralfocusing. Instead of using narrowband pulsed, one utilizes frequencychirped broadband pulses, i.e., laser pulses with varying centerfrequency over time. If the chirp rate of both pulses is matched, thefrequency difference is fixed and narrowband, even though the absolutefrequency is swept. By tuning the time delay, different differencefrequencies, i.e., different Raman peaks may be targeted. As suchspectral focusing may be used to achieve fast and reproducible spectralacquisition as desired for spectroscopic differentiation with CRS.Instead of a narrowband system. Systems of the invention may alsoprovide pulses for spectral focusing CRS by using broadband amplifiersand adjusting the chirp rate with a dispersion unit (e.g., opticalfibers with well-known dispersion). The delay may be scannedautomatically or manually with an optical delay stage.

Laser systems of various embodiments of the invention may also provideexcitation pulses for multiplex CRS. In multiplex CRS, either pump orStokes beam is broadband and narrowband, respectively. This means thatmultiple Raman transitions are excited simultaneously. By performingeither excitation or emission spectroscopy, it is possible to detect thesignal from each vibrations separately and simultaneously, i.e., in amultiplexed fashion. This means that multicolor images at multiplevibrations may be acquired. In another implementation of the disclosedlaser system multiplex CRS can be achieved by implementing a broadbandfirst train of pulses and a narrowband second train of pulses or viceversa.

As discussed above with reference to FIG. 4, either of the first andsecond trains of laser pulses may pass through an optical delay deviceto ensure coincident timing when the trains of laser pulses arecombined, and either the first train of laser pulses or the second trainof laser pulses may be modulated prior to combining the two trains oflaser pulses.

FIG. 21A shows at 510 the narrow bandwidth of the output spectra and thetuning range for the Erbium-doped amplifier, and FIG. 21B shows at 512the narrow bandwidth and tuning range for the Ytterbium-doped amplifier.

In accordance with further embodiments, the picosecond amplifier mayinclude a chirped amplifier system, and the illumination system may beused in a spectral focusing imaging system, which requires lasers with abandwidth smaller or comparable to the typical Raman linewidth. Ifbroadband lasers are chirped linearly however, (i.e., the instantaneouslaser frequency changes over time), and that the same rate for both pumpand Stokes, the difference frequency is essentially narrowband. Ratherthan tuning the difference by changing the center frequency of thelasers, CRI spectra may be acquired by tuning the time delay.

FIG. 22 shows that the frequency of each of the pump and Stokes fieldsmay be varied over time and frequency, yet provide the same differencefrequency as shown at 514 and 516 in FIG. 22. This provides for spectralfocusing. Specifically, it shows that an amplified frequency-shiftedtrain of pulses may be provided having pulses with a temporally evolvinginstantaneous frequency over the picosecond pulse duration.

FIG. 23, for example, shows a system in accordance with a furtherembodiment of the present invention that includes a laser system (asdiscussed above) that provides a first train of laser pulses to achirped control system 524, which provide the first output train oflaser pulses 526. The system 520 also includes a frequency shiftingsystem 528 (as discussed above) and a chirped amplifier system 530 thatprovides the second output train of laser pulses 532. The laser system522, the gain media, and the frequency shifting system 528, may be asdescribed above with reference to FIGS. 1-20.

As shown in FIG. 24, a system 540 in accordance with a furtherembodiment may include a laser system 542 and a chirped control system544 for providing the first train of laser pulses 546. The system 540may also include a frequency shifting system 548, as well as a chirpedamplifier system 550 that includes a chirp unit 552 (e.g., a fiber orprism pain) and an amplifier 554, which provides the second output trainof laser pulses 556. Again, the laser system 542, the gain media, andthe frequency shifting system 548, may be as described above withreference to FIGS. 1-20.

FIG. 25 shows a system 560 in accordance with a further embodiment thatincludes a laser system 562 and a chirped control system 564 forproviding the first train of laser pulses 566. The system 560 alsoincludes a frequency shifting system 568, as well as a chirped amplifiersystem 570 that includes a broadband amplifier 572, chirp unit 572(e.g., a fiber or prism pain) and an amplifier 574, which provides thesecond output train of laser pulses 576. Again, the laser system 562,the gain media, and the frequency shifting system 568, may be asdescribed above with reference to FIGS. 1-20.

FIG. 26 shows a system 580 in accordance with a further embodiment thatincludes a laser system 582 and a chirped control system 584 forproviding the first train of laser pulses 586. The system 580 alsoincludes a frequency shifting system 588, as well as a chirped amplifiersystem 590 that includes a broadband amplifier 592, an amplifier 594,frequency doubling or tripling unit 596, and a chirp unit 598 (e.g., afiber or prism pain), which provides the second output train of laserpulses 600. Again, the laser system 582, the gain media, and thefrequency shifting system 588, may be as described above with referenceto FIGS. 1-20, and the frequency doubling or tripling unit 596 may be asdiscussed above with reference to FIGS. 15 and 16.

FIG. 27 shows a system 610 in accordance with a further embodiment thatincludes a laser system 612 and a chirped control system 614 thatincludes a chirp unit 616 (e.g., fiber or prism pain) for providing thefirst train of laser pulses 618. The system 610 also includes afrequency shifting system 620, as well as a chirped amplifier system622, which provides the second output train of laser pulses 624. Again,the laser system, the gain media, 612 and the frequency shifting system620, may be as described above with reference to FIGS. 1-20.

FIG. 28 shows a system 630 in accordance with a further embodiment thatincludes a laser system 632 and a chirped control system 634 thatincludes a chirp unit 616 (e.g., fiber or prism pain) and an amplifier638 for providing the first train of laser pulses 640. The system 630also includes a frequency shifting system 642, as well as a chirpedamplifier system 644, which provides the second output train of laserpulses 646. Again, the laser system 632, the gain media, and thefrequency shifting system 642, may be as described above with referenceto FIGS. 1-20.

FIG. 29 shows a system 650 in accordance with a further embodiment thatincludes a laser system 652 and a chirped control system 654 thatincludes a broadband filter 656, a chirp unit 658 (e.g., fiber or prismpain) and an amplifier 660 for providing the first train of laser pulses662. The system 650 also includes a frequency shifting system 664, aswell as a chirped amplifier system 666, which provides the second outputtrain of laser pulses 668. Again, the laser system 652 and the frequencyshifting system 664, the gain media, may be as described above withreference to FIGS. 1-20.

FIG. 30 shows a system 670 in accordance with a further embodiment thatincludes a laser system 672 and a chirped control system 674 thatincludes a broadband filter 676, an amplifier 678, a frequency doublingor tripling unit 680, and a chirp unit 682 (e.g., fiber or prism pain)for providing the first train of laser pulses 684. The system 670 alsoincludes a frequency shifting system 686, as well as a chirped amplifiersystem 688, which provides the second output train of laser pulses 690.Again, the laser system 672, the gain media, and the frequency shiftingsystem 686, may be as described above with reference to FIGS. 1-20.

FIG. 31 shows at 700 an image taken using a CRS system with anillumination system in accordance with an embodiment of the presentinvention wherein the size is 1 μm, the sampling was 512 by 512 pixels,and the imaging speed was one frame per second (4 μs/pixel). FIG. 32shows at 710 a cross-section of the image 700 with a signal to noiseratio of greater than 25.

FIG. 33A shows at 720 an image taken using a CRS system with anillumination system in accordance with an embodiment of the presentinvention of CH2 vibrations (lipids) using a Stokes beam of 1018 nm, apump beam of 789 nm and a wavenumber of 2850 CM⁻¹. FIG. 33B shows at 722an image taken using a CRS system with an illumination system inaccordance with an embodiment of the present invention of CH3 vibrations(proteins) using a Stokes beam of 1028 nm, a pump beam of 789 nm and awavenumber of 2950 CM⁻¹. FIG. 34 shows at 730 a combination of theimages 720, 722 of FIGS. 33A and 33B.

Those skilled in the art will appreciate that numerous modifications andvariations may be made to the above disclosed embodiments withoutdeparting from the spirit and scope of the claims.

What is claimed is:
 1. An illumination system for providingdual-excitation wavelength illumination for non-linear opticalmicroscopy and micro-spectroscopy, said illumination system comprising:a laser system including a laser oscillator for providing a first trainof pulses at a center optical frequency ω₁; an optical splitting meansfor dividing the first train of pulses at the center optical frequencyω₁ into a first split train of pulses and a second split train ofpulses; a frequency shifting system for shifting the optical frequencyof the first split train of pulses to provide a frequency shifted trainof pulses; and a picosecond amplifier system for amplifying thefrequency shifted train of pulses to provide an amplifiedfrequency-shifted train of pulses having a pulse duration of at least0.5 picoseconds.
 2. The illumination system as claimed in claim 1,wherein said picosecond amplifier is a narrowband amplifier.
 3. Theillumination system as claimed in claim 2, wherein said narrowbandamplifier is tunable to provide that the difference frequency of theamplified frequency-shifted train of pulses and the second split trainof pulses may be varied.
 4. The illumination system as claimed in claim2, wherein said narrowband amplifier system includes a narrowbandfilter.
 5. The illumination system as claimed in claim 2, wherein saidnarrowband amplifier system includes a plurality of amplifiers inseries.
 6. The illumination system as claimed in claim 1, wherein saidlaser system includes a high power oscillator.
 7. The illuminationsystem as claimed in claim 1, wherein said laser system includes a lowpower oscillator and an amplifier.
 8. The illumination system as claimedin claim 1, wherein said illumination system further includes a laseramplifier for amplifying the second split train of pulses.
 9. Theillumination system as claimed in claim 1, wherein said frequencyshifting system includes a highly non-linear fiber.
 10. The illuminationsystem as claimed in claim 1, wherein said frequency shifting systemincludes a photonic crystal fiber.
 11. The illumination system asclaimed in claim 1, wherein said frequency shifting system furtherincludes a laser amplifier.
 12. The illumination system as claimed inclaim 1, wherein said picosecond amplifier is a chirp amplifier toprovide an amplified frequency-shifted train of pulses having pulseswith a temporally evolving instantaneous frequency over the picosecondpulse duration.
 13. The illumination system as claimed in claim 12,wherein said chirped amplifier system includes a chirp unit and anamplifier.
 14. The illumination system as claimed in claim 13, whereinsaid chirped amplifier system further includes a broadband filter. 15.The illumination system as claimed in claim 14, wherein system furtherincludes a chirp control system in the second split train of pulses. 16.The illumination system as claimed in claim 1, wherein system furtherincludes: a combiner for combining the amplified frequency-shifted trainof pulses and the second split train of pulses from the opticalsplitting means; and adjusting means for adjusting a time delay betweenthe amplified frequency-shifted train of pulses and the second splittrain of pulses with sub-picosecond precision to provide a collinearoutput of the dual-excitation wavelength illumination for the non-linearoptical microscopy or micro-spectroscopy system.
 17. The illuminationsystem as claimed in claim 1, wherein said laser system is one of aYtterbium fiber laser and an Erbium fiber laser.
 18. The illuminationsystem as claimed in claim 1, wherein said laser system is a TitaniumSapphire laser.
 19. The illumination system as claimed in claim 1,wherein said picosecond amplifier system is one of an Erbium fiberamplifier, a Ytterbium fiber amplifier, a Thulium fiber amplifier, and aHolmium fiber amplifier.
 20. The illumination system as claimed in claim1, wherein the output of the picosecond amplifier system is frequencydoubled (SHG) or tripled (THG).
 21. The illumination system as claimedin claim 1, wherein the second split train of pulses from the opticalsplitting means is frequency doubled (SHG) or tripled (THG).